Electrophotographic element comprising an organic photoconductive layer sensitized locally at its surface

ABSTRACT

The present invention relates to an electrophotographic element having a high sensitivity to visible light which comprises a conductive support and an organic photoconductive layer comprising a polymer and an organic sensitizer, said organic sensitizer being distributed locally at the top layer of said organic photoconductive layer.

' United States Patent 1 1 Ikeda et a1.

[ ELECTROPHOTOGRAPHIC ELEMENT COMPRISING AN ORGANIC PHOTOCONDUCTIVE LAYER SENSITIZED LOCALLY AT ITS SURFACE [75] Inventors: lvlitsusuke Ikeda, Hirakata; Hisanao Sato, Ibaragi; Etsuko Torii, Kyoto;

Kazuhisa Morimoto, Settsu; Yo Hasegawa, Suita, all of Japan [73] Assignee: Matsushita Electric Industrial Co.,

Ltd., Japan 22 Filed: Aug. 13, 1973 211 App]. No.: 387,696

Related US. Application Data [63] Continuation of Ser No. 185,657, Oct. 1, 1971,

abandoned.

{30] Foreign Application Priority Data Oct. 7, 1970 Japan 45-88433 Oct. 8, 1970 45-89152 Oct. 8, 1970 45-89153 Oct. 8, 1970 Japan 45-89154 Oct. 13, 1970 Japan 45-90353 [52] US. Cl. ..96/1.6; 117/34; 117/201 1 1 Oct. 14, 1975 Primary Exantiner-Norman G. Torchin Assistant Examiner.ludson R. l-lightower Attorney, Agent, or FirmWenderoth, Lind & Ponack [57] ABSTRACT The present invention relates to an electrophotographic element having a high sensitivity to visible light which comprises a conductive support and an organic photoconductive layer comprising a polymer and an organic sensitizer, said organic sensitizer being distributed locally at the top layer of said organic photoconductive layer.

4 Claims, 2 Drawing Figures US. Patent Oct. 14, 1975 Sheet 1 of2 3,912,508

Fig.1

ELECTROPHOTOGRAPHIC ELEMENT COMPRISING AN ORGANIC PHOTOCONDUCTIVE LAYER SENSITIZED LOCALLY AT ITS SURFACE This is a continuation, of application Ser. No. 185,657, filed Oct. 1, 1971, now abandoned.

This invention relates to an electrophotographic ele ment comprising organic materials, and more particularly to a novel electrophotographic element having a high sensitivity to visible light.

In electrophotography, an electrostatic latent image is formed by exposing a uniformly charged photoconductive layer to a light through or reflected from an original document. Said electrostatic latent image is developed by using a toner directly on the photoconductive layer or after being transferred to a dielectric layer, or is changed to a frost image by heating.

Selenium and zinc oxide are well known as inorganic photoconductive materials useful for electrophotography. Recently, much attention has been paid to organic photoconductive materials because of their excellent characteristics such as transparency, light weight, flexibility, and flat surface. However, the electrophotographic sensitivity of organic photoconductive materials is usually much lower than that of evaporated selenium photoconductive layer. The organic electrophotographic element having sensitizers distributed uniformly therein generally has a considerable residue potential in photo-decay process.

It is an object of this invention to provide an electrophotographic element having a high sensitivity to a visible light.

It is another object of this invention to provide a high sensitive electrophotographic element which is transparent and flexible.

It is still another object of this invention to provide an electrophotographic element which shows little residue potential in the photo-decay process.

Details of this invention becomes apparent upon consideration of the following description taken together with the accompanying drawings in which:

FlG. 1 shows a section through an electrophotographic element according to this invention.

FIG. 2 shows sensitivity curves for a conventional electrophotographic element and a novel electrophotographic element of this invention.

It has been discovered according to this invention that an electrophotographic element including a conductive support and an organic photoconductive layer comprising a polymer and an organic sensitizer, said organic sensitizer being distributed locally at the top layer of said organic photoconductive layer shows a high sensitivity to a visible light.

Referring to FIG. 1, a conductive support 1 supports an organic photoconductive layer 2 having a sensitizer distributed locally at the top layer 3 thereof. A layer 4 below said top layer 3 is named as a plain layer for convenience.

The conductive support 1 may comprise any suitable conductive support. For example, a flexible film having a cuprous iodide layer or an evaporated conductive metal layer thereon can be used when flexibility and/or transparency is required. And further, a tin oxidecoated conducting glass or metal plate can be used when flexibility and/or transparency is not required.

The organic photoconductive layer 2 comprises at least one polymer selected from poly-N-vinylcarbazole,

poly-N-alkenylcarbazole, and nuclear-substituted poly- N-vinylcarbazole, in accordance with the present invention. Addition of a plasticizer and polycarbonate into the organic photoconductive layer 2 is effective to improve mechanical properties of the layer 2. The plasticizer comprises chlorinated diphenyl, epoxy resin, chlorinated fatty acid ester, phthalic acid ester, or mixtures thereof. The amount of the plasticizer is less than weight parts, preferably 20 to 40 weight parts relative to weight parts of the polymer. Polycarbonate has the effect of improving the mechanical properties of the poly-N-vinylcarbazole, or nuclear-substituted poly-N-vinylcarbazole layer without reducing the transparency. The amount of polycarbonate to be added is less than 60 weight parts, preferably 10 to 40 weight parts relative to 100 weight parts of the polymer.

The organic photoconductive layer 2 has a thickness of 3 to 30 microns, preferably 5 to 20 microns. The top layer 3 having organic sensitizers distributed locally therein has a thickness of 0.1 to 1 micron, preferable 0.3 to 0.6 micron. The organic photoconductive layer 2 comprises 0.1 to 10 weight parts, preferably 0.4 to 2 weight parts of an organic sensitizer in the top layer 3 and 100 weight parts of a polymer.

The organic sensitizer comprises pyrylium salts described in U.S. Pat. No. 3,250,615, thiapyrylium salts described in U.S. Pat. No. 3,250,615, carbonium salts described in US. Pat. No. 3,575,698, benzopyrylium salts described in U.S. Pat. No. 3,526,502, benzopyrylium salt derivatives described in U.S. Pat. application 4200 filed Jan. 20, 1970, or mixtures thereof.

Among the many sensitizers described above, the more preferable are a reaction product of 2-(pmethoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-mehtoxystyryl)-3-phenyl-4-methoxypyrane, 2-[(2'-a-phenyl-p-methoxystyryl-4'- benzopyranylidene )methyl]-3-phenylbenzopyrylium perchlorate, and 2-( p-methoxystyryl 3 phenylbenzopyrylium perchlorate. They produce a top layer 3 having spectral response more matched with a tungsten lamp and a high sensitivity. The reaction product of 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl-4- methoxypyrane is obtained from the reaction of these in hot 1,2-dichloroethane solution described in U.S. Pat. application 4200 filed Jan. 20, 1970, and the chemical structure of the reaction product has not been identified.

Any suitable method for making this organic photoconductive layer can be used. One method is as follows: A solution including aforesaid polymer in any solvent such as toluene, chlorobenzene, or dioxane is applied to any suitable conductive support by any suitable and available method such as knife, blade, or reverse-roll coating method and is dried at a temperature of 40 to 100C for a time of more than 5 minutes to form a plain layer in a desired thickness. If necessary, the solution may further include a plasticizer and polycarbonate to improve mechanical properties of the plain layer. Another solution including aforesaid sensitizer and one monomer such as N-vinylcarbazole, N- alkenylcarbazole, or nuclear-substituted N- vinylcarbazole in a solvent inert to the plain layer is applied to the plain layer by any suitable method such as whirler, blade, or kiss coating method and is cured by heating at a temperature of 80 to C for a time of more than 5 minutes to form a top layer having the sensitizer destributed locally therein. It is necessary that the monomer corresponds to the material in the plain layer. In other words, when the plain layer includes poly-N-vinylcarbazole, the operable monomer is N- vinylcarbazole. After being cured, the monomer is changed to a polymer at the top layer. Thus, the top layer is well combined with the plain layer and forms a photoconductive layer according to the present invention. If necessary, said solution may further include a plasticizer and polycarbonate to improve mechanical properties of resultant photoconductive layer. A preferable solvent for use in another solution is alcohol such as methanol or ethanol, or ketone such as acetone or methylethylketone, or buthylacetate.

Another method is to deposite a sensitizer at the surface of the plain layer. A plain layer is first prepared by a method similar to that mentioned above and then is coated with aforesaid organic sensitizer by, for example, vacuum evaporation. The coating can be also achieved by applying a solution including aforesaid sensitizer in a solvent such as alcohol or ketone to the plain layer and by drying. The plain layer coated with the sensitizer is exposed to vapor such as that of toluene, chlorobenzene, or dioxane to dissolve both the sensitizer and the plain layer. The vapor controlled in density and exposure time dissolves the sensitizer and only a surface layer of the plain layer and forms a photoconductive layer having the sensitizer distributed locally at the top layer thereof in accordance with the present invention.

A further method is to use a discolored sensitizer in a solution. The discolored sensitizer can be obtained by treating aforesaid sensitizer with a reducing agent such as hydrogen or carbon monoxide and has no sensitizing action. The solution including the discolored sensitizer and aforesaid polymer such as poly-N-vinylcarbazole, poly-N-alkenylcarbazole, or nuclear-substituted poly- N-vinylcorbazole, if necessary, a plasticizer and polycarbonate in a solvent is applied to a conductive support by any suitable method and is dried. The dried material has the discolored sensitizer distributed uniformly therein and forms a photoconductive layer according to the present invention when treated, at the surface, with an oxidizing agent such as oxygen. The oxidizing agent causes the discolored sensitizer to recolor and to recover the sensitizing action. The thickness of the top layer having recovered sensitizer distributed therein is controlled by the treatmenttime and temperature.

In a conventional organic electrophotographic element, an organic photoconductive layer supported on a conductive support has organic sensitizers distributed uniformly therein. Even when the same polymer for example, poly-N-vinylcarbazole and the same organic sensitizer, for example, the benzopyrylium salt derivative, are used, the novel electrophotographic element has a higher sensitivity to visible light than that of the conventional organic electrophotographic element. The reason for the difference in the sensitivity between the novel electrophotographic element according to the present invention and the conventional electrophotographic element is not certain. A possible explanation is as follows: In the conventional electrophotographic element, the sensitizer distributed uniformly in the organic photoconductive layer acts as a trap to catch photocarriers. On the other hand, the sensitizer distributed locally at the top layer as shown in FIG. 1

has no bad effect on the movement of photo-carriers in the organic photoconductive layer beneath the top layer. In the element of this invention, photo-carriers generated at the top layer of the element with the help of sensitizers may drift to the conductive support without being trapped. Thus, when the element of this invention is charged to a positive potential and then is exposed to a visible light, the surface potential of the element dissipates rapidly and shows little residual potential. A conventional electrophotographic process for forming an electrostatic latent image on the element of this invention comprises a process of charging the element of this invention to a positive potential and exposing it to a light image. Said latent image is developed by using a toner directly on the surface of said element or after being transferred to a suitable dielectric layer, or is changed to a frost image by heating.

Photosensitivity of the element is defined as a halfdecay exposure, said exposure is the exposure to reduce a surface potential of the element to a half of the surface potential in the dark. And further, the exposure to reduce a surface potential of the element to 20 percent of the surface potential in the dark is defined as 20 percent-decay exposure which also represents the photosensitivity. The smaller the half-decay or 20 percentdecay exposure, the more photosensitive the element.

Various embodiments of this invention will be shown in the following examples and should not be construed as limitative.

EXAMPLE 1 An electrophotographic element is first prepared. 1.0 gram of poly-N-vinylcarbazole (Luvican M-l70, available from BASF Company) and 0.5 gram of chlorinated diphenyl (Kanechlor No. 400, available from Kanegafuchi Chemical Inductrial Company) are dissolved in 10 milli-liter of toluene to form a solution. The solution is applied to an aluminum plate by blade coating and dried at 60C for 5 minutes to obtain a plain layer of about 10 microns in thickness. An acetone solution containing l.0 gram of N-vinylcarbazole, 0.5 gram of chlorinated diphenyl and 0.1 gram of the reaction product of 2-(p-methoxystyryl)-3- phenylbenzopyrylium perchlorate and 2-(pmethoxystyryl)-3-phenyl-4-methoxypyrane is applied to the plain layer and is heated at C for 30 minutes to form a top layer of about 0.5 micron in thickness. The thus obtained electrophotographic element is then charged to a positive or a negative potential of about 800 volts by corona discharge means and exposed to a tungsten lamp. The decay curve of the surface potential of the element is measured by an electrometer and is shown in FIG. 2. As shown in FIG. 2, the element of this invention has a photoresponse when charged to a positive potential (curve A) but has no photoresponse when charged to a negative potential (curve C). Halfdecay exposure of the element is 5 lux seconds.

For comparison, a conventional electrophotographic element is prepared. A solution containing 1.0 gram of poly-N-vinylcarbazole, 0.5 gram of chlorinated diphenyl, and 5 milligram of the sensitizer the same as above in 10 milliliters of toluene is applied to an aluminum plate by blade coating and dried at 60C for 5 minutes to obtain a photoconductive layer of about 10 microns in thickness. The amount of the sensitizer contained in said photoconductive layer is the same as that in the top layer of the above novel' electrophotographic element. The thus obtained conventional electrophotographic element having the sensitizer distributed uniformly throughout the photoconductive layer is then charged to a positive or a negative potential of about 800 volts by corona discharge means and exposed to a tungsten lamp (2800K). Measured decay curve of the surface potential is shown in FIG. 2. This conventional element has a photoresponse when charged to a positive and a negative potential (curve B). Half-decay exposure of the element is 15 lux secondsfThus, it is evident that the element of this invention is' much more photosensitive than the conventional element.

In FIG. 2, surface potential of the element is indicated on the vertical axis in volt units, while exposure is indicated along the horizontal axis in lux second units.

EXAMPLE 2 Electrophotographic elements are prepared as in Example 1 except that here the sensitizer used is 1 2,4-diphenyl-6-(a-amyl-3 ',4'-dimethoxystyryl)- pyrylium fluoborate.

2. tris-p-methoxyphenylcarbonium perchlorate, or

3 2- (2 -a-phenyl-p-methoxystyryl-4 benzopyranylidene)methyl]-3- phenylbenzopyrylium perchlorate. and heating of the top layer of the element is performed at 100C for 60 minutes. Half-decay exposure of these elements is shown in Table I.

For comparison, conventional elements having sensitizers distributed uniformly throughout the photoconductive layer are prepared as in Example 1 except that here the sensitizers used is the same as above and halfdecay exposure of these is also shown in Table I.

Elements are prepared as in Example 1 except that here the amount of the sensitizer at the top layer is varied from I to 100 weight parts relative 100 weight parts of N-vinylcarbazole(NVl(). Half-decay exposure of these elements is shown in Table II.

Table II The amount of a sensitizer at the top layer versus half-decay exposure The amount of a sensitizer at the top layer (in weight Half-decay parts relative to I00 weight exposure (in lux parts of NVK) seconds) 1 8O 2 28 4 l8 7 7.6 10 6.0 8.0 8.6 30 I0 50 I4 100 26 EXAMPLE 4 Electrophotographic elements are prepared as in Example 1 except that here heating of the top layer having sensitizers therein is performed at a temperature of 60 to 160C for 30 minutes or at 120C fora time of 5 to 60 minutes. I V I Heating effect is ,shown in Tables III and IV in relation to half-decay exposure of these elements. It is evident that the sensitivity of the element of this invention is enhanced by heating. Thesensitivityof the conventional element, however, can not be enhanced by heating.

' Table III Heating effect of the top layer at a temperature of 60 to 160C for 30 minutes Heating temperature Half-decay exposure Table IV Heating effect of the top layer having Sensitizers at 120C for a time of 5 to 60 minutes Time of heating Half-decay exposure (in minutes) (in lux seconds) EXAMPLE 5 An element is prepared as follows. 1.0 gram of brominated poly-N-vinylcarbazole (monobromosubstituted compound described in U.S. Pat. No. 3,421,891), 0.3 gram of polycarbonate resin. (Panlite L, available from Teijin Compa y), and 0.3 gram of epoxy resin (Epicote 828, available from Shell Oil Company) are dissolved in a mixed solvent of 9 milliliters of chlorobenzene and 3 milliliter of 1,2- dichloroethane to form a solution. The solution is applied to a transparent conductive polyester film having cuprous iodide thereon by blade coating and is dried at C for 20 minutes to obtain a plain layer of about 8 microns in thickness. An organic top layer containing sensitizers of about 0.6 micron in thickness is formed on the plain layer by applying an acetone solution containing 1.0 Gram of 3,6-dibromo-N-vinylcarbazole and 0.1 Gram of the sensitizer the same as in Example 1 and by heating at C for 10 minutes. This element is transparent and flexible. Half-decay exposure of the el- EXAMPLE 6 An element is prepared as follows. A plain layer of about 10 microns in thickness is formed on an aluminum plate as in Example 1. A layer of about 0.1 micron in thickness is formed on the plain layer by the evaporation of tris-p-methoxyphenylcarbonium perchlorate, and then is exposed to vapor of dichloromethane and dried. 1 v

Half-decay exposure of this element is lux seconds by a visible light illumination.

What is claimed is:

1. An electrophotographic element including a conductive support and an organic photoconductive layer comprising 100 weight parts of a polymer and 0.1 to 10 weight parts of an organic sensitizer, said organic photoconductive layer having a thickness of 5 to 30 microns, said organic sensitizer being distributed locally at the top of said organic photoconductive layer in a layer with a thickness of 0.1 to 1 micron, said polymer comprising a member selected from the group consisting of poly-N-vinylcarbazole, poly-N-alkenylcarbazole, nuclear-substituted poly-N-vinylcarbazole, and mixtures thereof, said organic sensitizer comprising a member selected from the group consisting of pyrylium salts, thiapyrylium salts, carbonium salts, benzopyrylium salts, benzopyrylium salt derivatives, and mixtures thereof. 7

2. An electrophotographic ,element according to clai'rnl, wherein said organic photoconductive layer is formed by overcoating of a solution comprising said organic sensitizer, a monomer corresponding to said polymer and a solventonto a polymer layer comprising said polymer, said solvent comprising a member selected from the group consisting of methanol, ethanol, acetone, methyl-ethylketone and butylacetate.

-3. An electrophotographic element according to .claim 1, whereinsaid organic sensitizer is a member selected from the group consisting of 2-[2-a-phenyl-pmethoxystyryl-4 -benzopyranylidene)methyl1-3- phenylbenzopyrylium perchlorate and 2-(pmethoxystyryl)-3-phe nylbenzopyrylium perchlorate.

- 4. An electrophotographic element according to claim 1, said organic sensitizer is the reaction product of Z-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate and 2( p-methoxystyryl )-3-phenyl-4- methoxypyrane. 

1. AN ELECTROPHOTOGRAPHIC ELEMENT INCLUDINGG A CONDUCTIVE SUPPORT AND AN ORGANIC PHOTOCONDUCTIVE LAYER COMPRISING 100 WEIGHT PARTS OF A POLYMER AND 0.1 TO 1/ WEIGHT PARTS OF AN ORGANIC SENSITIZER, SAID ORGANIC PHOTOCONDUCTIVE LAYER HAVING A THICKNESS OF 5 TO 30 MICRONS, SAID ORGANIC SENSITIZER BEING DISTRIBUTED LOCALLY AT THE TOP OF SAID ORGANIC PHOTOCONDUCTIVE LAYER IN A LAYER WITH A THICKNESS OF 0.1 TO 1 MICRON, SAID POLYMER COMPRISING A MEMBER SELECTED FROM THE GROUP CONSISTING OF POLY-N-VINYLCARBAZOLE, POLY-N-ALKENYLCARBAZOLE, NUCLEARSUBSTITUTED POLY-N-VINYLCARBAZOLE, AND MIXTURES THEREOF, SAID ORGANIC SENSITIZER COMPRISING A MEMBER SELECTED FROM THE GROUP CONSISTING OF PYRYLIUM SALTS, THIAPYRYLIUM SALTS, CARBONIUM SALTS, BENZOPYRYLIUM SALTS, BENZOPYRYLIUM SALT DERIVATIVES, AND MIXTURES THEREOF.
 2. An electrophotographic element according to claim 1, wherein said organic photoconductive layer is formed by overcoating of a solution comprising said organic sensitizer, a monomer corresponding to said polymer and a solvent onto a polymer layer comprising said polymer, said solvent comprising a member selected from the group consisting of methanol, ethanol, acetone, methyl-ethylketone and butylacetate.
 3. An electrophotographic element according to claim 1, wherein said organic sensitizer is a member selected from the group consisting of 2-(2''- Alpha -phenyl-p-methoxystyryl-4''-benzopyranylidene)methyl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate.
 4. An electrophotographic element according to claim 1, said organic sensitizer is the reaction product of 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl-4-methoxypyrane. 